前言
在openwrt上配置了opnvpn只能上网不能访问内网机器，本想装个tcpdump排查一下，发现一直如下报错。
Package libwebsockets-openssl version 2.4.1-2 has no valid architecture, ignoring.
Package libwrap version 7.6-1 has no valid architecture, ignoring.
Package libxml2 version 2.9.9-1 has no valid architecture, ignoring.
Package sbc version 1.3-2 has no valid architecture, ignoring.
Unknown package 'tcpdump'.
Collected errors:
以为是tcpdump，不支持该硬件设备，想一圈，安卓都支持，怎么可能不支持这个。开始排查。最终定位到软件源问题。
解决
刚开始换源，以为随便搜个源就能用，发现根本不行。树莓派正常的源可用的源如下
src/gz openwrt_core https://mirrors.tuna.tsinghua.edu.cn/openwrt/releases/18.06.0/targets/brcm2708/bcm2710/packages
src/gz openwrt_base https://mirrors.tuna.tsinghua.edu.cn/openwrt/releases/18.06.0/packages/aarch64_cortex-a53/base
src/gz openwrt_luci https://mirrors.tuna.tsinghua.edu.cn/openwrt/releases/18.06.0/packages/aarch64_cortex-a53/luci
src/gz openwrt_packages https://mirrors.tuna.tsinghua.edu.cn/openwrt/releases/18.06.0/packages/aarch64_cortex-a53/packages
src/gz openwrt_routing https://mirrors.tuna.tsinghua.edu.cn/openwrt/releases/18.06.0/packages/aarch64_cortex-a53/routing
src/gz openwrt_telephony https://mirrors.tuna.tsinghua.edu.cn/openwrt/releases/18.06.0/packages/aarch64_cortex-a53/telephony
想换别的源，需要注意如下几点：
- 18.06.0版本要对，和openwrt版本一直。
- 架构要对，如aarch64_cortex-a5，比如你是x86的就需要换别的。具体怎么换，访问上面的url，找到对应的架构目录即可。
- 换完之后，执行opkg update更新一下，然后重新安装即可解决问题。
